UN High Commissioner for Refugees Filippo Grandi attends a news conference on Myanmar at the United Nations in Geneva, Switzerland, Sept 27, 2017. Reuters

A quick agreement to return Rohingya refugees to Myanmar is looking less and less likely, the UN refugee chief said on Friday as he launched an almost $1 billion appeal to support the stateless Muslims in camps in Bangladesh for the rest of 2018.

The United Nations says 671,000 Rohingya fled from Myanmar's Rakhine state after militant attacks in August sparked a crackdown led by security forces, which a UN investigator said bore "the hallmarks of genocide".

Filippo Grandi, head of the UN refugee agency UNHCR, said the Rohingya must be allowed to return to their homes in Myanmar, but returns could only take place voluntarily and if their rights were guaranteed.

“The odds are growing against a solution, an early solution anyway, but this is not a reason for me as High Commissioner for Refugees to give up on this issue,” he told a news conference.

"We are very mindful of the fact that this situation may take a long time, and therefore we need to plan for this."

Asked how long the Rohingya might stay in their camps in Bangladesh at the minimum, Grandi said: “I wish I knew.”

Last year he had visited another part of Myanmar where displaced people had been stuck in camps for six or seven years. He said the Rohingya could not be subjected to the same fate on a much larger scale.

Myint Thu, permanent secretary at Myanmar's Ministry of Foreign Affairs, said on Wednesday that a first batch of 374 refugees could be repatriated.

He also said Myanmar now considered it was appropriate to invite UNHCR and the UN Development Programme into the repatriation process.

Grandi said he still did not have agreement for UNHCR's involvement, which he said was "crucial, essential and indispensable".

"The discussions with Myanmar have been pretty basic, not very frequent, not very advanced, but they have continued. This is the way it goes in this situation. It’s very difficult,” Grandi said.

The government does not recognise the Rohingya as citizens, and Grandi said their rights needed to be established, including citizenship, freedom of movement and access to services.

But first of all the violence had to stop. Although movements into Bangladesh have decreased massively since the flood late last year, they continue to come in smaller numbers, suggesting the situation has still not stabilised, he said.

He also said that other governments should only invest in development projects in Rakhine if they ensured it supported the presence of Rohingyas and did not perpetuate the situation.